<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div,
    p {
      width: 300px;
      height: 30px;
      line-height: 30px;
      color: deeppink;
      background-color: aquamarine;
    }
  </style>
</head>

<body>
  <button id="btn">显示当前系统事件</button>
  <div>某个时间</div>
  <p>易烊千玺吖</p>
  <script>
    var btn = document.getElementById('btn');
    var div = document.querySelector('div')
    btn.onclick = function () {
      // div.innerText = '2012-5-3';
      div.innerText = getDate();
    }

    function getDate () {
      var date = new Date();
      var year = date.getFullYear();
      var month = date.getMonth() + 1;
      var dates = date.getDate();
      var arr = ['星期日', '星期一', '星期二', '星期三', '星期四', '星期五', '星期六'];
      var day = date.getDay();
      return '今天是' + year + '年' + month + '月' + dates + '日' + arr[day];
    }

    // 元素可以不添加事件 
    var p = document.querySelector('p');
    p.innerText = getDate();  // 不点击就直接显示
  </script>
</body>

</html>